What is a condensing gas fired boiler?

A condensing gas fired boiler can condense the water vapour in the flue gases and withstand the corrosive and acidic qualities of the flue gas condensate. Although most boilers can condense the flue gasses, only boilers that have heat exchangers constructed from materials able to withstand the corrosion should be used in condensing application. Condensing gas fired boilers with primary and secondary heat exchanger do not work well as they are not able to fully condense and defeat the purpose of using a condensing boiler. Also, boilers with this arrangement are prone to condensation in the primary heat exchanger at lower firing rate which can cause damage to the heat exchanger.